Wheel rims and tethers
Put simply, the rim is the connection point between the tyre, which is the only part of the car to make contact with the road, and the chassis itself. Without a rim you could not bolt a tyre to the car, as even if the rubber was moulded to allow it, the consistency of rubber would not provide a good solution. Furthermore, wheel rims also have an aerodynamic importance, as cooling air from the brakes passes through the rim openings form the inside of the wheel to the outside. Hence, it's important to have as little side surface of the rims as possible to enhance brake cooling.
The FIA has however clamped down considerably on the design options for wheel suppliers following the push in 2009 and 2010 from teams to fit carbon fibre fairings on the outside of the wheels to enhance cooling and control the airflow in the wake of the tyres (particularly important for the front wheels, as the wake has a considerable impact on the car's rear end downforce generation).
More recently, teams have worked together closely with rim suppliers to speed up the changing of wheels and tyres during pitstops. Since 2010 there is a tendency to create advanced wheel nuts that can lock the wheels in place (by means of a pneumatic fastener) while at the same time apply the safety pin to prevent the wheels from running off when not securely fastened. Going along that route, by 2012 nearly all teams had integrated the wheel nut into the rim, ensuring that the nut is always at their perfect location already when positioning a wheel onto the car.
While there used to be total freedom in wheel design, strict regulations now mandate the exact alloys that can be used to construct wheels for Formula One cars. And apart from the obvious wheel dimension specifications to ensure everybody can use the same tyres there are strict limitations on almost every measurable dimension of a wheel.
12.3 Wheels must be made from AZ70 or AZ80 magnesium alloys.
12.4 Wheel dimensions :
12.4.1 Complete wheel width must lie between 305mm and 355mm when fitted to the front of the car and between 365mm and 380mm when fitted to the rear.
12.4.2 Complete wheel diameter must not exceed 660mm when fitted with dry-weather tyres or 670mm when fitted with wet weather tyres.
12.4.3 Complete wheel width and diameter will be measured horizontally at axle height, with the wheel held in a vertical position and when fitted with new tyres inflated to 1.4 bar.
12.4.4 Wheel dimensions and geometry must comply with the following specifications :
- The minimum wheel thickness is 3.0mm.
- The minimum bead thickness is 4.0mm (measured from hump to outer edge of the lip).
- The ETRTO standard bead profile is prescribed.
- The tyre mounting widths are 12” (304.8mm +/-0.5mm) front; 13.7” (348.0mm +/-0.5mm) rear.
- The wheel lip thickness is 9mm (+/-1mm).
- The outer lip diameter is 358mm (+/-1mm).
- A lip recess of maximum 1.0mm depth between a radius of 165mm and a radius of 173mm from wheel axis is permitted (for wheel branding, logo, part number, etc).
- With the exception of the wheel lip, only a single turned profile with a maximum thickness of 8mm is allowed radially outboard of the exclusion zones specified in Article 12.4.5.
- The design of the wheel must meet the general requirements of the tyre supplier for the mounting and dismounting of tyres including allowance for sensors and valves.
- The wheel design cannot be handed between left and right designs.
The wheel thickness specifications are mainly in place for strength and safety, as thin layers of magnesium are highly flammable and could hence be a threat for the driver's safety in case of an accident.
Article 12.4.5 continues to specify a number of areas where it is not allowed to have any wheel material, followed by article 12.4.6 which effectively outlaws wheel fairings.
12.4.6 When viewed perpendicular to the plane formed by the outer face of the wheel and between the diameters of 120mm and 270mm the wheel may have an area of no greater than 24,000mm2.
The team either buys their wheels from a supplier or gets them for free in a sponsorship deal. Some teams even have their say in the development and the eventual specifications of the wheel rims, as they too have an effect on the total performance of a car.
Each of the rim manufacturers has their own design of wheel rim and approach the challenge of making an F1 tyre rim in a slightly different way so there are subtle differences between the rims. “The shape of the wheel rim has many design parameters. It has to allow the fitting of the tyre. It has to hold the tyre and allow an airtight seal to be created. It has to be light. It has to be as thin as possible to allow the biggest brakes to be fitted beneath it. It has to give good heat transfer and dispersion. It has to withstand tremendous forces without deforming. It has to enable good airflow to the brakes, but not create unwanted aerodynamic drag and its shape and width affect the spring rate of the tyre itself.
As well as holding the tyre, there are other components which are mounted on a Formula One wheel rim. Of course, there is the valve, which allows the tyre to be filled and deflated. Then there is the tyre pressure monitor. Sometimes there are other valves too, which allow the tyres to be purged.
Once at the track, teams deliver their bare wheel rims to the tyre manufacturers truck where the tyres are put onto the rims with special machines. The tyres are then inflated and delivered back to the teams.
Hirohide Hamashima, Director of Bridgestone Motorsport Tyre Development explained: "We use a fitting paste when we are fitting the tyres to the rims, but it is the deformation of the rubber at the tyre edge where it meets the rim that creates the air-tight seal and ensures that the tyre retains its pressure."
Since 1998, F1 cars have had to fit wheel tethers connecting the wheels to the chassis. This rule was introduced to try to stop wheels coming free and bouncing around dangerously during an accident. Unfortunately, wheels still do come off the cars during crashes, tragically killing a marshall at the Italian GP in 2000. The FIA have introduced an extra tether to each wheel for the 2001 season to try to stop the wheels coming off and causing injury to other drivers, marshals or spectators. The tether must attach to the chassis at one end, with the other end connecting to the wheel hub.
The wheel tethers are made by one of three companies in the UK, Future Fibres in London being one of them, and take the form of a rope. The tethers used in F1 are a derivative of high performance marine ropes, made especially for each car. They are made from a special polymer called polybenzoaoxide (PBO) which is often called Zylon. This Zylon material has a very high strength and stiffness characteristic (around 280GPa) much like carbon, but the advantage of Zylon is that it can be used as a pure fibre unlike carbon which has to be in composite form to gain its strength. The drawback of Zylon is that is must be protected from light, so it is covered in a shrink wrapped protective cover. The tethers are designed to withstand about 5000 kg of load, but often they can break quite easily during an accident, especially if the cable gets twisted by the broken suspension members. The teams normally replace the tethers every two or three races to ensure that they can withstand the loads put on them during an accident.
Thanks to Bridgestone